1. Field of the Invention
The present invention relates to an element which converts electric energy into light to emit light, and, particularly, to an organic electroluminescent element which is preferably used in the fields of display elements, back lights, lighting sources, electrophotographic exposure systems, marks, signboards and the like.
2. Description of the Related Art
Electroluminescent elements are spontaneous light emitting all-solid elements, have high visibility and are strong against impacts and are therefore expected to be used in wide applications. At present, electroluminescent elements employing inorganic phosphors are dominantly and widely used. However, these electroluminescent elements are problematic in that the cost of running them is high due to the high a.c. voltage of 200 V or more required to power them, and further, they do not provide sufficient brightness.
Studies on electroluminescent elements using organic compounds were first started using a single crystal such as anthracene. However, the thickness of a film was as thick as about 1 mm, requiring a drive voltage of 100 V or more. For this, an attempt has been made to make a thin film by a vapor deposition method (Thin Solid Films, Vol. 94, 171 (1982)). The emission of light from such an electroluminescent element is a phenomenon that electrons are injected from one of the electrodes of the electroluminescent element and holes are injected from the other electrode, whereby the light emitting material in the electroluminescent element is excited to a higher energy level and then emits excess energy as light when the excited light emitting body is returned to the ground state. However, the drive voltage required is still as high as 30 V and also the densities of electron and hole carriers are low. Therefore, because the probability of photons being created by carrier recombination is low, only insufficient brightness can be obtained and these electroluminescent elements using organic compounds have not been put to practical use.
In the meantime, a function-separating type organic electroluminescent element which was produced by laminating a hole-transferable organic low-molecular compound and a fluorescent organic low-molecular compound having electron-transferability as very thin films in this order on a transparent substrate and ensured a brightness as high as 1000 cd/m2 or more even under a voltage as low as about 10 V was reported by Tang et al., in 1987 (Appl. Phys. Lett., Vol. 51, 913 (1987) and Japanese Patent Application Laid-pen (JP-A) No. 59-194393). Organic electroluminescent elements having a laminate structure have been researched and developed actively since then.
Such an organic electroluminescent element has a structure in which an organic luminous body and a charge-transferable organic material (charge transfer material) are laminated on an electrode, wherein the holes and electrons of each material move in the charge transfer material and are recombined with each other to emit light. As the organic luminous body, organic dyes and the like, such as an 8-quinolinol aluminum complex and cumalin compound, which emit fluorescent light are used. Also, examples of the charge transfer material include diamino compounds such as N, N-di (m-tolyl)-N, N′-diphenylbenzidine and 1, 1-bis[N, N-di(p-tolyl) aminophenyl] cyclohexane and 4-(N, N-diphenyl) aminobenzaldehydo-N, N-diphenylhydrazone compounds.
However, the aforementioned organic electroluminescent elements have high light emitting characteristics but have problems concerning heat stability when light is emitted and preserving stability. The thickness of the organic material layer constituting the above organic electroluminescent element is extremely thin, from several dozen to several hundred nanometers in thickness, and the voltage applied per unit thickness is therefore increased. As a consequence, because the element is driven at a current density as high as several mA/cm2, it generates a large amount of Joule heat. For this, a hole-transferable low-molecular compound and a fluorescent organic low-molecular compound which are formed as films in the form of an amorphous glass state by vapor deposition are gradually crystallized and finally fused, exhibiting deteriorated heat stability. Such deteriorated heat stability causes a reduction in brightness and dielectric breakdown, with the result that such a problem arises that the life of the element is shortened. Also, a change with time when the element is used for a long period of time is effected and a deterioration is caused by an oxygen-containing atmosphere and moisture.
In light of this, in order to solve the problem concerning heat stability, organic electroluminescent elements are reported which use a star burst amine which can provides a hole transfer material in a stable amorphous glass state (The 40th meeting, Japan Society of Applied Physics & Related Societies Preprints 30a-SZK-14 (1993)) or use a polymer in which triphenyl amine is introduced into the side chain of polyphosphazene (The 42nd Polymer meeting, Preprints 20J21 (1993)). However, when each of these elements is used independently, it satisfies neither the ability of injecting holes from an anode nor the ability of injecting holes into a light emitting layer because of the presence of an energy barrier caused by the ionizing potential of the hole transfer material. Also, in the case of the former star burst amine, there is the problem that it has less solubility and thus difficult to refine and therefore difficult to raise the purity. In the case of the latter polymer, there is the problem in that high current density cannot be obtained and the available brightness is therefore insufficient.
On the other hand, the research and development of organic electroluminescent elements having a monolayer structure are being undergone. Elements using a conductive polymer such as poly(p-phenylenevinylene) (for example, Nature, Vol. 357, 477 (1992)) and elements obtained by compounding an electron transfer material and a fluorescent dye in hole-transferable polyvinylcarbazole (The 38th meeting, Japan Society of Applied Physics & Related Societies Preprints 31p-G-12 (1991)) are proposed. However, these elements are far behind laminate type organic electroluminescent elements using an organic low-molecular compound in brightness, luminous efficacy and the like.
Also, it has been reported that as to production methods, a coating system is preferable from the viewpoint of the simplification of production, processability, an increase in area and cost and the element can be obtained also by a casting method (The 50th meeting, Japan Society of Applied Physics, Preprints 29p-ZP-5 (1989) and The 51st meeting, Japan Society of Applied Physics, Preprints 28a-PB-7 (1990)). However, because the charge transfer material is inferior in solubility and compatibility with solvents and resins, it tends to crystallize easily and therefore, from the viewpoint of production, does not possess suitable qualities.